Multilayer adsorbent beds for PSA gas separation

ABSTRACT

The invention comprises a PSA process and apparatus wherein the fixed adsorbent bed comprises an equilibrium zone and a mass transfer zone. Further, the equilibrium and mass transfer zones each comprise at least one adsorbent material, selective for the adsorption of a more selectively adsorbable component, that is selected on the basis of the performance of that adsorbent material under the process conditions applicable to either the equilibrium or mass transfer zones.

FIELD OF THE INVENTION

The invention relates to pressure swing adsorption (PSA) processes andapparatus, more particularly to the use of high performance adsorbentsin PSA processes and systems through the novel deployment of suchadsorbents in layers.

BACKGROUND

Cryogenic methods have dominated air separation processes for many yearswhere high purity O₂, N₂ and/or Ar are desired. More recently, bothmembrane and adsorption processes have become important commercially. Inparticular, PSA, including superatmospheric adsorption/desorptionprocesses, subatmospheric vacuum swing adsorption (VSA) andtransatmospheric vacuum pressure swing adsorption (VPSA) processes arewell known in the art. Such methods are typically used to produce oxygenhaving a purity between about 90 to 95%. There is an increasing need forthis purity O₂ in such diverse industries as steel making, glass makingand pulp and paper production. Single plant oxygen capacity for suchadsorption processes now exceeds 100 tons-per-day contained O₂ (TPDO),and applications continue to arise demanding even greater capacities. Atthese production and purity levels, O₂ product cost is lower byadsorption than by cryogenic methods, while for larger capacities,economies of scale currently favor the cryogenic methods. Nevertheless,there continues to be considerable economic incentive to extend theproduction range of adsorption processes for air separation. This mustbe accomplished by improving performance while reducing the cost ofpower and capital.

A typical adsorption system for the production of O₂ includes one ormore adsorber vessels containing a layer of pretreatment adsorbent forremoving atmospheric contaminants followed by a main adsorbent. Thepretreatment adsorbent can be any material primarily effective inremoving H₂ O and CO₂, e.g. zeolites, activated alumina, silica gel,activated carbon and other such adsorbents. The main adsorbent material,which usually represents at least 90% of the total volume of adsorbentin the vessel is N₂ -selective, typically from the type A or type Xfamily of zeolites. While many different adsorption cycles have beendeveloped for O₂ production, all pressure swing cycles contain the fourbasic steps of pressurization, adsorption, depressurization anddesorption. When multiple beds are used, the beds are sequenced out ofphase for the different cycle steps in order to maintain a constant flowof product. One of many examples of such processes illustrating thesebasic features is given by Batta in U.S. Pat. No. 3,636,679.

There has been significant development of the various PSA, VSA and VPSAmethods for air separation over the past thirty years, with majoradvances occurring during the last decade. Commercialization of theseprocesses and continued extension of the production range can beattributed primarily to improvements in the adsorbents and processcycles, with advances in adsorber design contributing to a lesserdegree. Highly exchanged lithium molecular sieve adsorbents, asillustrated by Chao in U.S. Pat. No. 4,859,217, are representative ofadvanced adsorbents for O₂ production. A historical review of bothadsorbent and process cycle development may be found in Kumar (Sep. Sci.and Technology, 1996).

The increase in N₂ /O₂ selectivity and N₂ working capacity associatedwith N₂ -selective advanced adsorbents is largely responsible for theimprovements in O₂ recovery and reduction in power and bed size factor(BSF). Such adsorbents, however, often have higher heats of adsorption,are more difficult to manufacture and may have poorer mass transfercharacteristics, all resulting in a higher adsorbent cost. While manynew adsorbents have been developed claiming improved properties for airseparation, only a few have been implemented successfully in commercialprocesses. Advanced adsorbents often fail or fall short of expectationssince process performance is projected on the basis of adsorbentequilibrium properties and isothermal process conditions.

Collins in U.S. Pat. No. 4,026,680 teaches that adiabatic operationintensifies the thermal effects in the adsorbent bed inlet zone. Inparticular he teaches that there is a "sharply depressed temperaturezone," (hereinafter referred to as a "cold zone"), in the adsorption bedinlet end. This zone is as much as 100° F. below the feed gastemperature. Such a zone results in a thermal gradient over the lengthof the adsorbent bed of approximately the same magnitude (e.g. about100° F.). Collins suggests that the cold zone arises from the couplingof an "inadvertent heat-regenerative step" at the inlet end of the bedwith the thermal cycling resulting from the adsorption/desorption stepsof the process. The regenerative effect may be partly the result of theadsorption of water vapor and carbon dioxide in a pretreatment zonelocated ahead of the main adsorbent.

The thermal cycling that occurs in an adiabatic process results in anadverse thermal swing, i.e. the adsorption step occurs at a highertemperature than the desorption step. This thermal swing tends toincrease with increasing adsorbate/adsorbent heats of adsorption andincreasing ratio of adsorption to desorption pressure. These gradientsand swings in bed temperature result in various parts of the adsorbentbed functioning at different temperatures. The N₂ /O₂ selectivity and N₂working capacity of any particular adsorbent may not be effectivelyutilized over such wide ranges in bed temperature. Dynamic adsorbentproperties that vary strongly with temperature are also likely to resultin process instability when operating conditions, such as ambienttemperature, change.

Considerable attention has been given to eliminating or minimizing thecold zone in adiabatic adsorbers since Collins. Earlier suggestionsincluded raising the feed temperature using external heating or throughpartial bypass of the feed compressor aftercooler.

Collins proposed the use of heat conducting rods or plates extending thelength of the bed for the same purpose. Others have extended thisconcept by replacing the rods or plates with hollow tubes filled withliquid to provide heat transfer by convection between the warmer productend and the colder feed end of the adsorber. For example, the cold zonetemperature is increased from -70° C. to near 0° C. in Fraysse et al.(U.S. Pat. No. 5,520,721) by supplying a heat flux to a passage betweenthe pretreatment and main adsorbents. The primary intent in all of thesemethods is to elevate the minimum temperature near the feed inlet of theadsorber using direct and/or indirect heat exchange. The entire bedtemperature is elevated along with the cold zone temperature when thefeed is heated, however, and the overall size of the thermal gradient inthe bed remains relatively unaffected.

Another approach attempts to match an adsorbent with a temperature thatis most efficient for the desired separation. Typical of such teachings,an adsorbent bed is divided into layers that are maintained at differenttemperatures using embedded heat exchangers to affect distinctseparations.

Armond (EP 0512781 A1) claims to inhibit the effect of the cold zone byselecting two unspecified adsorbents with high removal efficiency forN₂, at -35° C. to -45° C. and at ambient temperature, respectively. Thelow temperature material is located near the feed inlet (but downstreamof the pretreatment adsorbent) and is followed by the second material.

A main adsorbent, containing at least two layers, has been disclosed byWatson et al. (U.S. Pat. No. 5,529,610) for O₂ production. Watsonteaches that no commercially available adsorbent functions optimallyover the large temperature gradient (as much as 100° F.) that exists inthe main adsorbent region of the bed. NaX zeolite, comprising from 20%to 70% of the total adsorbent volume, is chosen for the lowesttemperature region of the bed due to its low capacity and highselectivity at such temperatures. The second layer is preferably CaXzeolite, although other high capacity, high nitrogen selectivityadsorbents are also proposed for this region.

Co-pending and commonly assigned application Ser. No. 08/546,325, nowU.S. Pat. No. 5,674,311, to Leavitt et al. discloses layered beds inwhich the adsorbents are selected according to optimum adsorptionfigures-of-merit (AFM) at particular temperatures in the bed. Thefigure-of-merit index is computed from equilibrium properties of theadsorbent. As with the teachings cited above, Leavitt teaches that oneshould address large thermal gradients (e.g. about 70° F.) in anadsorber.

Reiss teaches in U.S. Pat. No. 5,114,440 a VSA process for O₂ enrichmentof air using two or three layers of CaA zeolite of varying N₂ capacityfor the main adsorbent. The CaA adsorbents are arranged such that thematerial of lowest N₂ capacity is placed near the feed inlet while thatof highest N₂ capacity is located near the product end of the adsorber.Power consumption was shown to be lower for the layered CaA adsorber ascompared to adsorbers containing CaA of uniform N₂ capacity and anadsorber containing NaX near the feed inlet followed by CaA near theproduct end.

JP Appl. No. 4-293513 teaches that improved stability of operation (lessvariability in bed size factor (BSF), power, and final desorptionpressure) is achieved under varying ambient temperatures (-10° C. to 40°C.) in VPSA O₂ production using a layered main adsorbent bed consistingof equal volumes of CaA and CaX zeolites when compared to adsorberscontaining either of the individual adsorbents alone. The CaA zeolite islocated near the feed end and is followed by the CaX adsorbent.

Multiple adsorbent layers have also been proposed in order to reduce theoverall cost of product O₂. Such an approach is disclosed in U.S. Pat.No. 5,203,887 (Toussaint), wherein a layer of less costly NaX replacesLiX adsorbent in a section of the main adsorbent nearest the productend. An alternative to this two-layer arrangement for the main adsorbentis the addition of a third layer (NaX) between the LiX and thepretreatment layer near the feed inlet of the adsorber.

Thus, the prior art has focused upon mitigating the apparent undesirableeffects of the subambient cold zone through heat transfer means and/orby selection of an appropriate adsorbent for the low temperature regionof the bed. Layering of adsorbents has been proposed as a means ofimproving separation efficiency in the presence of large bed temperaturegradients (50-100° F.). While the most commonly suggested adsorbents forthe cold zone are NaX and CaA zeolites, a variety of adsorbents havebeen recommended for the regions of the bed beyond the cold zone.

OBJECT OF THE INVENTION

It is therefore an object of the invention to provide a PSA process andapparatus that achieve improved efficiency, reduced cost and extendedproduction ranges for PSA air separation processes using advancedadsorbents.

It is a further object of the invention to provide a PSA process andsystem having no cold zone and consequently small (e.g. less than about50° F.) temperature gradients.

It is a further object of the invention to provide a PSA apparatusrequiring no additional equipment for heat addition or removal from theadsorber.

SUMMARY OF THE INVENTION

The invention comprises a PSA process and apparatus wherein the fixedadsorbent bed comprises an equilibrium zone and a mass transfer zone.Further, the equilibrium and mass transfer zones each comprise at leastone adsorbent material, selective for the adsorption of a moreselectively adsorbable component, that is selected on the basis of theperformance of that adsorbent material under the process conditionsapplicable to said zone.

In a preferred embodiment, at least one adsorbent material selected forthe equilibrium zone is selected on the basis of said adsorbentmaterial's adiabatic separation factor for a gas mixture of two or morecomponents.

In another preferred embodiment, at least one adsorbent materialselected for either the equilibrium zone or the mass transfer zone isselected on the basis of said adsorbent material's adiabatic separationfactor for a gas mixture of two or more components.

In another preferred embodiment, at least one adsorbent materialselected for either the equilibrium zone or the mass transfer zone isselected in view of the different gas compositions in said zones duringat least one of adsorption or desorption.

In still another preferred embodiment, at least one adsorbent materialselected for the mass transfer zone has a comparatively high adiabaticseparation factor for the more adsorbable material and a comparativelylow adiabatic delta loading for the less adsorbable component under theprocess conditions applicable to said zone.

In another preferred embodiment, the gas mixture is air.

It should be noted that the terms "working capacity", "dynamic capacity"and "delta loading" as used herein are interchangeable. Also for thepurposes of this invention, the property referred to by these terms isdetermined under adiabatic operation.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled inthe art from the following description of preferred embodiments and theaccompanying drawings, in which:

FIG. 1 is a schematic diagram of an embodiment of the invention whereinthe overall structure of a fixed adsorbent bed is set forth; the arrowsindicate the direction of gas flow through the bed during adsorption;

FIG. 2a is a graph showing adsorbent bed temperature across the bed atthe end of adsorption and desorption;

FIG. 2b is a graph showing the oxygen loading in mmol/g across thelength of the same adsorbent bed at the end of adsorption anddesorption;

FIGS. 3 and 5 are graphs showing the variation of adiabatic separationfactor with the bed temperature (wherein the temperature is measured atthe end of adsorption), for a series of adsorbents at a pressure ratio(adsorption pressure:desorption pressure) of 5;

FIG. 4 is a graph showing the variation of adiabatic nitrogen workingcapacity (e.g. adiabatic delta nitrogen loading) with the bedtemperature (wherein the temperature is measured at the end ofadsorption), for a series of adsorbents;

FIG. 6a is a graph showing the variation in delta oxygen loadings as afunction of oxygen content in the mass transfer zone of the adsorbentbed, for a series of adsorbents;

FIG. 6b is a graph showing the variation in adiabatic separation factoras a function of oxygen content in the mass transfer zone of theadsorbent bed, for a series of adsorbents; and

FIG. 7 is a graph showing the variation of adiabatic separation factorwith bed temperature (wherein the temperature is measured at the end ofadsorption), for a series of adsorbents at a pressure ratio (adsorptionpressure:desorption pressure) of 3.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides improved efficiency, reduced cost andextended production ranges for PSA, VSA and VPSA air separationprocesses using advanced adsorbents to produce oxygen having a purity ofbetween about 90% to 95% by volume. An essential factor in the inventionis the recognition that thermal gradients in adsorbers employing suchadvanced materials are much smaller (about 34-36° F.) than thosedisclosed in the prior art (typically about 100° F.). According to theinvention, improved PSA performance is achieved if adsorbents areselected and layered in equilibrium and mass transfer zones in anadsorbent bed on the basis of adiabatic selectivity and adiabaticworking capacity at the prevailing temperatures and feed gascompositions in each zone. This represents a complete departure fromprior art systems, wherein adsorbents were selected on the basis oftheir equilibrium behavior at particular temperatures, and on theobservation of high thermal gradients in adsorbent beds.

Further, because adsorbents are selected in the present invention suchthat the adsorber has no cold zone, additional costly equipment is notrequired for heat addition or removal from the adsorber as in the priorart. Several Li-exchanged type X adsorbents, including those containingmixed cations, have been identified to achieve such results.

The prior art has given much attention to increasing the heavy component(N₂) capacity of the adsorbent. Further, it has recognized that it isalso important to reduce the coadsorption of the light component (O₂) inorder to maximize recovery. Light component coadsorption becomes morepronounced at low temperatures (e.g. <270K) for some adsorbents.

The prior art has not recognized, however, that a large percentage ofthe total amount of coadsorbed light component is contained in aparticular region of an adsorbent bed, e.g. the mass transfer zone.Consequently, as the cycle time decreases, lower light product recoveryoccurs as the fixed size of the mass transfer zone becomes an increasingfraction of the overall bed size. The invention represents a departurefrom the prior art in that it takes all of these factors: temperature,oxygen coadsorption, and mass transfer zone, into consideration in theselection of adsorbent materials for an adsorbent bed.

The invention may be accomplished through the deployment of adsorbents,as shown in FIG. 1, into three distinct adsorption zones in an adsorbentbed 1: pretreatment zone 2, equilibrium zone 3 and mass transfer zone 4.Adsorbents are selected for the latter two zones 3 and 4 on the basis ofadiabatic selectivity and adiabatic working capacity at the prevailingtemperatures and compositions in each zone. In addition, through theinvention, the coadsorption of O₂ may be minimized, particularly in themass transfer zone.

A typical temperature distribution in an adsorber containing onlyhighly-exchanged LiX adsorbent, operating in a commercial level O₂production process, has been derived and is illustrated in FIG. 2a. Atthe end of adsorption, the total temperature gradient in the mainadsorbent is less than 16° K (29° F.). This example is typical for highperformance N₂ -selective adsorbents (such as LiX) for which there maybe no significant cold zone and the entire bed operates with a modestthermal gradient (e.g. less than 50° F.). An ideal adsorbent for theequilibrium zone is one possessing high adiabatic N₂ /O₂ selectivity,high adiabatic N₂ dynamic capacity and good thermal stability in thedesired operating range of temperature and pressure.

The O₂ loading distributions in the main adsorbent at the end ofadsorption and at the end of desorption steps are shown in FIG. 2b. Thedifference between these two distributions (shown as the shaded area inFIG. 2b) represents the amount of coadsorbed O₂ or adiabatic dynamic O₂capacity in an actual adiabatic process. This coadsorbed O₂, largelylost in the desorption step along with the waste N₂, is the primaryfactor effecting the O₂ recovery, and consequently the efficiency, ofthe process. As is evident from FIG. 2b, between 40% and 60% of thecoadsorbed O₂ is contained in the mass transfer zone. Thus, theappropriate adsorbent for the mass transfer zone should have highadiabatic selectivity for N₂ /O₂ and low adiabatic dynamic O₂ capacity(e.g. low adiabatic delta oxygen loading) in a region of increasingtemperature and O₂ concentration.

Since the prior art has selected main adsorbent(s) upon the basis of aneffectiveness evaluated at equilibrium conditions, thus effectivelytreating the entire bed as an equilibrium zone, the resulting amount ofcoadsorbed O₂ has been significantly underestimated from that whichoccurs in the real process. In addition the selection of adsorbents inthe prior art has been made without consideration of the adversetemperature swing that occurs between adsorption and desorptionconditions. Furthermore, while the prior art has taught the use ofdifferent adsorbents in order to optimize efficiency over very largethermal gradients in the adsorbent bed, these gradients can, in fact, besignificantly minimized through the practice of the present invention.

The invention recognizes that one should separate the main adsorbentinto equilibrium and mass transfer zones, and select adsorbents on thebasis of their adiabatic selectivity and adiabatic working capacity atthe different conditions that occur in each zone. Deployment ofadsorbents according to this invention has resulted in an increase inthe O₂ recovery and productivity and a decrease in BSF and powercompared to prior art systems.

As indicated above, adsorbents are deployed by the method of thisinvention in three distinct adsorption zones as illustrated in FIG. 1.One or more adsorbents may be contained in each zone. The pretreatmentzone 2 is nearest the feed inlet and its purpose is to remove anyundesirable contaminants from the feed stream. Typical contaminants inair separation are water and carbon dioxide. Those skilled in the artwill appreciate the use of zeolites, activated alumina, silica gel,activated carbon as well as other appropriate adsorbents in thepretreatment zone. The equilibrium 3 and mass transfer 4 zones containadsorbent(s) selective for the primary heavy components in the feed.These are the main adsorbents.

The method of adsorbent evaluation is important to the selection of mainadsorbents for the equilibrium 3 and mass transfer 4 zones. Theobjective is to estimate the separation behavior of an adsorbent underactual process conditions. This is accomplished by defining adiabaticselectivity (e.g. separation factor) and adiabatic working (e.g.dynamic) capacity as given in Equation (1). As applied below a binaryair feed composition is exemplified. ##EQU1##

In Equation (1), the amount of adsorbate or loading (L_(i)) is evaluatedfor each constituent at the end of the adsorption and desorption stepsat the temperature (T₁,T₂), pressure (P_(H), P_(L)) and composition(y_(i) (in mole fraction)) prevailing in the individual zones. The termsin the numerator and denominator of Equation (1) represent the heavy(N₂) and light (O₂) component adiabatic working capacities,respectively. This evaluation is accomplished using any appropriatemulticomponent isotherm model such as the loading ratio correlation setforth in Yang, Gas Separation by Adsorption Processes, 1987). Thoseskilled in the art will appreciate that the use of such a model requiresrepresentative adsorption data for the adsorbent and gas components ofinterest.

For example, the temperature swing (T₁ -T₂) must be determined fromeither experiment or adiabatic process simulation, (e.g. see in FIG. 2).Equation (1) is then applied to determine the variation in separationfactor with temperature in the equilibrium zone. Adsorption (P_(H)) anddesorption (P_(L)) pressures of 1.5 bar and 0.3 bar, respectively, wereused in the examples of FIGS. 2-6.

In addition, the highly lithium exchanged forms of zeolite X that areused in the preferred practice of the invention comprise zeolite Xadsorbent having a framework SiO₂ /Al₂ O₂ molar ratio not greater than 3and having at least 88%, of its AlO₂ tetrahedral units associated withlithium cations, with preferably at least 95% of said AlO₂ tetrahedralunits being associated with lithium cations. More preferably, saidlithium exchange is from about 95% to about 97%, or above. Such specialadsorbent materials include other materials in which the SiO₂ /Al₂ O₃molar ratio is from 2.0 to 2.5. These adsorbents are described in detailin Chao (U.S. Pat. No. 4,859,217).

Referring to FIG. 3, the adiabatic separation factor was determined forCaA (5A MG (medical grade), NaX (13X), LiX (SiO₂ /A₂ O₂ =2.3) and LiX(SiO₂ /Al₂ O₂ =2.0) adsorbents based on bed temperatures at the end ofthe adsorption step.

FIG. 4 sets forth the adiabatic N₂ working capacity for the sameadsorbents.

It is evident from the results set forth in FIGS. 3 and 4 that the LiXadsorbents have as much as twice the adiabatic N₂ working capacity andnearly 1.5 times the adiabatic selectivity compared to the conventionalCaA and NaX adsorbents. In light of this, LiX adsorbents are the mostpreferred material (of those compared) for the equilibrium zone whentemperatures in the bed are greater than about 270° K. In addition, themodest selectivity variation of the LiX adsorbents in this temperaturerange implies good process thermal stability (e.g. the change inadiabatic separation factor with temperature is minimal).

Further, FIG. 2a shows that at the end of the adsorption step a LiX(2.3) adsorbent bed, for example, has a temperature between about 300° Kand 320° K. From FIG. 3 it is clear that in this same temperature rangeLiX materials have a significantly higher adiabatic separation factorthan either NaX or CaA. As such, separation is maximized at everyposition in the equilibrium zone of the bed. Thus, as compared to theprior art, there is no need to alter the bed temperature or gradientusing additional heat transfer devices.

FIGS. 3 and 4 show that NaX has superior adiabatic selectivity andadiabatic N₂ working capacity at temperatures below 270° K, however, thethermal stability will be low due to the fact that the separation factordeclines steadily with increasing temperature.

The variation of adiabatic separation factor with temperature forseveral other adsorbents has been compared to that of LiX (2.0) in FIG.5. The CaLiX adsorbents (less than 30% Ca) also show promise for theequilibrium zone, particularly for bed temperatures above 300° K.Illustrative of such high Li-content adsorbents are those described byChao et al. (U.S. Pat. No. 5,174,979). Compared to LiX (2.0), theadiabatic N₂ working capacity of CaLiX (2.0) is slightly greater whilethat for CaLiX (2.3) is 20% to 40% lower. The CaLiX (2.0) materialappears to have better thermal stability while CaLiX (2.3) has higheradiabatic selectivity for temperatures above 320° K.

The adsorption of feed gas components occurs in the mass transfer zone,thus this is a region of continuously varying gas composition. In manyadsorption processes, the mass transfer zone forms rapidly and movesthrough the adsorbent at a steady rate. Combining the selection anddeployment of the proper adsorbent with appropriate operating conditionsresults in retention of the heavy component in preference to the lightcomponent in such a way that the desired separation is affected.

The purity of the light component increases in the mass transfer zonefrom feed concentration at the rear of the zone to the productconcentration at the zone front. It is generally in the interest ofmaintaining acceptable product purity to stop the adsorption step priorto the breakthrough of the mass transfer zone at the product end of thebed as described in Batta (U.S. Pat. No. 3,636,679).

At the end of the adsorption step that part of the adsorbent nearest thefeed end is in equilibrium with the feed composition, temperature andpressure. Ideally, the remainder of the adsorbent near the product endcontains just the mass transfer zone.

The condition described above is reflected in FIG. 2b, where theequilibrium and mass transfer zones are quite distinguishable. As showntherein, in the case of air separation, a considerable fraction of thepotential O₂ product is retained in the mass transfer zone at the end ofthe adsorption step. The efficiency of the process can be improved ifthis retained O₂ is minimized by selecting adsorbents in accordance withthe teachings of the invention, i.e. selecting an adsorbent with minimumadiabatic O₂ working capacity, but high N₂ /O₂ selectivity for the masstransfer zone.

The adsorption requirement in the mass transfer zone is quite differentthan in the equilibrium zone. In the equilibrium zone, it is desirableto remove and discharge as much heavy component as possible whileminimizing the amount of light component adsorbed. For air separation,the 4:1 N₂ /O₂ composition ratio of the feed in the equilibrium zone isadvantageous for the separation. This advantage is lost in the masstransfer zone as the mole fraction of O₂ in the feed increases from 0.21to 0.90, i.e. the adsorption of O₂ is greatly enhanced as itsconcentration exceeds that of N₂. Thus, high adiabatic N₂ workingcapacity is not as important as low adiabatic O₂ working capacity in themass transfer zone, while high adiabatic N₂ /O₂ selectivity is essentialto maintaining product purity and minimizing the size of the masstransfer zone. The temperature variation in this zone is small and muchless important than the gas composition change as can e inferred fromFIG. 2. The most suitable adsorbents for the mass transfer zone can bedetermined by applying Equation (1) and these criteria.

The adiabatic separation factor and adiabatic O₂ working capacity weredetermined as a function of O₂ mole fraction for seven adsorbents asshown in FIGS. 6a and 6b. This evaluation was performed at a temperatureof 320° K at the end of the adsorption step in conjunction with avariable temperature swing (e.g. the temperature swing decreased withincreasing O₂ mole fraction in the mass transfer zone as shown in FIG.2). Adsorption and desorption pressures were 1.5 bar and 0.3 bar,respectively.

It is evident from FIG. 6a that LiX (2.3), CaLiX (2.3), CaA and NaXadsorbents all have lower O₂ retention over the entire mass transferzone compared to LiX (2.0). Since LiX is a preferred adsorbent in theequilibrium zone, each of these materials substituted into the masstransfer zone may provide an improvement over an adsorber containing LiX(2.0) in both zones.

However, high adiabatic N₂ /O₂ selectivity must also be maintained inorder to minimize the size of the transfer zone. CaLiX (2.3) bestsatisfies the combined mass transfer zone criteria of reduced adiabaticO₂ working capacity and high adiabatic selectivity as shown in FIGS. 6aand 6b. The properties of this adsorbent are superior for the masstransfer zone relative to those of LiX (2.0). Conversely, the adiabaticseparation factors for NaX and CaA are substantially lower than theother adsorbents shown in

FIG. 6b. Consequently, NaX and CaA are not good choices for the masstransfer zone. LiX (2.3) has a lower adiabatic O₂ working capacity and aslightly lower selectivity than LiX (2.0). This adsorbent is stillexpected to show improvement when used in the mass transfer zone inconjunction with LiX (2.0) in the equilibrium zone as compared to LiX(2.0) in both zones. The increased adiabatic O₂ working capacities anddecreased adiabatic separation factors of CaX (2.0) and CaNaX (2.0),relative to LiX (2.0), are exactly opposite to the desired properties inthe mass transfer zone, and thus should not be used.

The methods and examples described above provide a means for selectingthe most effective adsorbents for the equilibrium and mass transferzones in the adsorber. It is expected that such selections will satisfythe objective of improved process performance. In order to verify thisexpectation, a computer model was applied to simulate adiabatic VPSA O₂processes for various deployments of adsorbents in the equilibrium andmass transfer zones. O₂ recovery and productivity, power and BSF weredetermined from these simulations.

EXAMPLE

Non-layered adsorbers containing only LiX (2.3) or LiX (2.0) and layeredadsorbers containing LiX (2.0) in the equilibrium zone and either LiX(2.3) or CaLiX (2.3) in the transfer zone were investigated. The totalamount of adsorbent was the same in all adsorbers. For the purpose ofthis example, the adsorbent in the mass transfer zones of the layeredbeds represented 25% of the main adsorbent volume which corresponds tothe approximate size of the mass transfer zone in a non-layered LiXadsorber operating under similar conditions.

The process conditions included a feed molar flux of approximately 17moles/m² ·second, a feed pressure of 1.5 bar, ambient temperature of 70°F., and a final desorption pressure of 0.3 bar. A basic VPSA cycle wasused which included adsorption, pressure equalizations, evacuation,purge and repressurization with feed. The model represented a two-bedsystem (nominal 60 TPDO capacity) where the two beds operate in paralleland out of phase with each other. A nominal 60s cycle was used, althoughcycle time was varied slightly between the test cases to maintain O₂product purity at 90%. Process performance was normalized for allconfigurations to the performance of the adsorber containing only LiX(2.3). The results are compared in the table below.

    ______________________________________                                                               LiX (2.0) +                                                                              LiX (2.0) +                                          LiX   LiX     LiX (2.3)  CaLiX                                                (2.3) (2.0)   75/25      75/25                                       ______________________________________                                        O.sub.2 Recovery                                                                         1       1.01    1.05     1.07                                      O.sub.2 Productivity                                                                     1       1.01    1.05     1.07                                      BSF        1       0.96    0.94     0.92                                      Power      1       0.99    0.97     0.95                                      ______________________________________                                    

The modest improvement in process performance of the non-layered LiX(2.0) over that of LiX (2.3) is consistent with the expectations of theadiabatic separation factor and adiabatic N₂ working capacity results ofFIGS. 3 and 4 for the bed temperature range of 300 K to 320 K. The lowerBSF of the LiX (2.0) results from the higher adiabatic N₂ workingcapacity of this adsorbent. The layered configuration of LiX (2.0) withLiX (2.3) in the mass transfer zone resulted in improvements in O₂recovery and productivity of 5% and a reduction in BSF of 6% compared tothe LiX (2.3) non-layered adsorber. The LiX (2.0)/CaLiX (2.3)combination was even better, with 7% improvements in O₂ recovery andproductivity and an 8% reduction in BSF. In all cases the unit power wasreduced as a result in the increase in O₂ recovery. It is noted thatwhile the examples above describe only a single adsorbent for each ofthe two main adsorbent zones, the invention is not limited to such aconfiguration.

One skilled in the art of adsorption will appreciate that the relativesizes of the equilibrium and mass transfer zones varies according to thecomponents to be separated, the process conditions and the adsorbentproperties. Thus, this invention is not limited to a fixed ratio ofadsorbents for the two zones for a given type of separation. On thecontrary, the ratio of adsorbents shall be the same as the ratio of thesizes of the equilibrium and mass transfer zones that exist at the endof the adsorption step. Methods for estimating the size of each zone arewell known in the art. For example, one may use process simulations andthe results obtained therefrom as illustrated in FIG. 2.

In the practice of the invention, it is conceivable that the deploymentof several different adsorbents in the equilibrium zone as layers mayprovide the optimum adiabatic selectivity and adiabatic N₂ workingcapacity, depending upon the thermal conditions within the zone. It mayalso be preferred to layer more than one type of adsorbent across thelight component concentration gradient in the mass transfer zone inorder to reduce the total adiabatic oxygen delta loading in that zone.When there are more than two components to be separated, more than asingle main adsorbent may be required, i.e. each main adsorbent zone mayconsist of an equilibrium zone followed by a mass transfer zone for eachcomponent separation to be affected.

Another feature of the present invention is the selection of advancedadsorbents for improved efficiency of heavy component removal in small(e.g. about 30° F.) to moderate (e.g. about 35-50° F.) thermalgradients. On the one hand, such adsorbents generally have a strongeraffinity for the heavy component, a higher heat of adsorption and agreater thermal swing. On the other hand, higher separation efficiencyis achieved for these stronger absorbents operating at loweradsorption/desorption pressure ratios than weaker adsorbents operatingat higher pressure ratios. Lower pressure ratios favor reducedtemperature swings and smaller bed thermal gradients. While the examplesgiven so far represent modest bed thermal gradients for a pressure ratioof 5.0, even smaller gradients and temperature swings are achieved atlower pressure ratios. By lower pressure ratios we mean: from about 1.4to about 4 for subatmospheric and transatmospheric processes, and fromabout 1.4:1 to 2.5:1 for superatmospheric processes.

Adsorbent evaluations and selection for deployment in adsorption zonesas demonstrated above has been repeated for lower adsorption/desorptionpressure ratios. As a non-limiting example, the adiabatic separationfactors are compared for several adsorbents at a pressure ratio of 3.0(P_(H) =1.5 bar, P_(L) =0.5 bar) in FIG. 7. This comparison applies toconditions in the equilibrium zone for the same adsorbents shown in FIG.5.

As can be seen from FIG. 7, the separation factors for these adsorbentsdecreased at the lower pressure ratio, but the relative performance ofthe various adsorbents remained about the same as in FIG. 5. Similarresults were obtained for the mass transfer zone. Consequently, theselection of adsorbents for the two zones remained unchanged at thelower pressure ratio for this group of materials, although thetemperature range of application shifts a small amount in some cases.

Layered beds containing highly-exchanged LiX and mixed cation LiXadsorbents have been shown to provide improved VPSA O₂ productionefficiency and thermal stability in the bed temperature range of 280 Kto 320 K. There will be, however, conditions such as ambient temperatureextremes, that force operation outside this range of bed temperatures.In such cases, other adsorbents can be used in the equilibrium zonealone or along with LiX (2.0) or LiX (2.3). For example, in highertemperature operations, a layer of LiX adsorbent would be used in thatpart of the equilibrium zone at bed temperatures less than 320 Kfollowed by a layer of one of the CaLiX mixed cation adsorbents assuggested in FIG. 5 and FIG. 7 for higher temperatures.

When the temperature near the feed end of the bed is below 270 K, theresults of FIG. 3 suggest NaX adsorbent followed by LiX in theequilibrium zone. The amount of NaX in the equilibrium zone must be keptto a minimum of less than 15%, preferably less than 10% of the mainadsorbent volume, because of the thermal instability of this adsorbent.Larger fractions of NaX in the equilibrium zone are likely to result infurther amplification of the cold region and the formation of the deepcold zones typical of the prior art.

Finally, other high lithium-exchanged adsorbents (Li only and mixedcation varieties) are likely to be applicable to air separation.Deployment of such adsorbents in layers according to the presentinvention is expected to provide significantly improved processefficiency for those adsorbents. Some examples of such adsorbents aredisclosed in Chao et al. (U.S. Pat. No. 5,174,979). There are many othersuch examples.

The present invention is particularly well-suited to cycle times of lessthan about two minutes and bed depths of less than about six feet inlength where the mass transfer zone is a larger fraction of the totaladsorbent bed size. Furthermore, it is understood that the layeringconcepts set forth in this invention apply equally well in axial flow,radial flow, lateral flow and other such fixed bed arrangements. Theinvention in its various embodiments may employ adsorption pressures upto about 1 or about 2 bar, and desorption pressures from about 0.25 toabout 1.0 bar.

Specific features of the invention are shown in one or more of thedrawings for convenience only, as such feature may be combined withother features in accordance with the invention. Alternative embodimentswill be recognized by those skilled in the art and are intended to beincluded within the scope of the claims.

What is claimed is:
 1. A process for the separation of a moreselectively adsorbable component from a gas mixture including a lessselectively adsorbable component, wherein said gas mixture is contactedwith an adsorbent bed, wherein said adsorbent bed comprises anequilibrium zone and a mass transfer zone, and wherein the equilibriumand mass transfer zones each include at least one adsorbent material,wherein said at least one adsorbent material is selective for theadsorption of said more selectively adsorbable component, and whereinsaid at least one adsorbent material included in said mass transfer zonehas a comparatively high adiabatic separation factor for the moreadsorbable material and a comparatively low adiabatic delta loading forthe less adsorbable component under the process conditions applicable tosaid mass transfer zone; and wherein said at least one adsorbentmaterial included in said equilibrium zone has a comparatively highadiabatic separation factor for the more adsorbable component andcomparatively high adiabatic dynamic capacity for the more adsorbablecomponent under the process conditions applicable to said equilibriumzone.
 2. The process of claim 1, wherein the more adsorbable componentis nitrogen and the less adsorbable component is oxygen.
 3. The processof claim 1, wherein said adsorbent material included in the masstransfer zone is CaLiX and the adsorbent material included in theequilibrium zone is LiX.
 4. The process according to claim 1, whereinsaid adsorbent bed further comprises a pretreatment zone, and whereinsaid pretreatment zone comprises a material selected from the groupconsisting of zeolites, activated alumina, activated carbon and silicagel.
 5. The process of claim 4, wherein the pretreatment zone comprisesless than about 12% of the total adsorbent volume in said adsorbent bed.6. The process of claim 1, wherein said adsorbent material included inthe equilibrium zone and the mass transfer zone is a LiX material. 7.The process of claim 1, wherein said adsorbent material in theequilibrium zone is LiX having a silica/alumina ratio of 2.0 and saidadsorbent material in the mass transfer zone is LiX having asilica/alumina ratio of 2.3.
 8. The process of claim 1, wherein said gasmixture is air.
 9. The process of claim 1, wherein said adsorbent bed isselected from the group consisting of axial flow beds, radial flow bedsand lateral flow beds.
 10. The process according to claim 1, whereinsaid equilibrium zone and said mass transfer zone have an adversetemperature swing between adsorption and desorption, and wherein said atleast one adsorbent material included in either the equilibrium zone orthe mass transfer zone is included in view of said adverse temperatureswing in either the equilibrium zone or the mass transfer zone.
 11. Theprocess of claim 1, wherein at least one of said process conditions isgas composition.
 12. An adsorbent bed wherein said bed comprises anequilibrium zone and a mass transfer zone, and wherein the equilibriumand mass transfer zones each comprise at least one adsorbent material,selective for the adsorption of a more selectively adsorbable componentfrom a mixture including a less selectively adsorbable component, andwherein said at least one adsorbent material included in the masstransfer zone has a comparatively high adiabatic separation factor forthe more adsorbable material and a comparatively low adiabatic deltaloading for the less adsorbable component under the process conditionsapplicable to said mass transfer zone; and wherein said at least oneadsorbent material included in said equilibrium zone has a comparativelyhigh adiabatic separation factor for the more adsorbable component and acomparatively high adiabatic dynamic capacity for the more adsorbablecomponent under the process conditions applicable to said equilibriumzone.
 13. The adsorbent bed of claim 12, wherein at least one of saidprocess conditions is gas composition.